The present invention relates to a method for identifying and if necessary blocking a fault current in a frequency converter. The invention also includes a frequency converter for generating an AC voltage with a predefinable alternating frequency from a mains voltage with a fixed alternating frequency.
The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.
An electrical machine can be operated in an electrical mains power supply for instance with a frequency converter. By means of the frequency converter an alternating current can be generated in stator windings of the machine in such cases, and in this process a frequency of the AC voltages of the individual motor cables can be set. To this end, both the frequency and also the amplitude of the AC voltage is predetermined by the AC converter by means of semiconductor power switches from the dc voltage of the intermediate circuit in each of the phase conductors on the basis of a pulse width modulation for instance. On account of the pulse width modulation used, fault currents may result during normal operation, i.e. currents which are not closed within the network but instead by way of a ground potential or chassis potential. In such cases the leakage currents can flow via a capacitive coupling between the arrangement comprising the frequency converter and the electrical machine on the one hand and objects in its surroundings on the other hand.
If the arrangement is to be able to be operated by way of a ground fault interrupter for personal safety and/or fire protection, the problem may arise in that such an interrupter cannot distinguish between an actual fault current (caused by a damaged cable for instance) and a leakage current generated during operation.
It would therefore be desirable and advantageous to obviate prior art shortcomings and to provide an improved ground fault interrupter in a frequency converter.